Diffusion-coating of nickel-base superalloy articles

ABSTRACT

A nickel-base superalloy article is provided with an oxidationresistant coating by heating it to 1,050*C for 3 hours in a bed consisting of a mixture of fine particles of FeAl3 (40%), NH4F (0.2%) and Al2O3 under a hydrogen atmosphere.

filtrate States Patent 11 1 1111 3,37,901 Seyboit 1 Sept. 24, 1974 [54]DIFFUSION-COATING OF NICKEL-BASE 3,257,230 6/1966 Wachtell 117 107.2 ROY ICL 3,415,676 12/1968 Nishi et a1 117/107.2 3,436,249 4/1969 Lambertet a1. 117/130 Inventor: Alan Seybolt, Ballston p NY 3,544,348 12/1970Boone 117/1072 P 3,677,789 7/1972 Bungardt et a1. 117/130 [73] Assgnee'g ggg gg fi e 3,771,974 11 1973 Itakura et a1. 117/107.2 P

[22] Filed: Aug. 21, 1970 Primary Examiner-Charles E. Van Horn AssistantExaminer-J. W. Massie [21] Appl' 66069 Attorney, Agent, or FirmCharlesT. Watts; Joseph T.

Cohen; Jerome C. Squillaro [52] US. (31..., 117/107.2 P, 106/1, 117/131[51] Int. Cl. C23c 9/02 [58] Field of Search 117/107.2 P, 130, 131; [57]T 106/62, 1 A nlckel-base superalloy arttcle 1s provlded wtth anoxidation-resistant coating by heating it to 1,050C for 5 ReferencesCited 3 hours in a bed consisting of a mixture of fine parti- UNITEDSTATES PATENTS cles of FeAl (40%), NH F (0.2%) and A1 0 under a hydrogenatmosphere. 3,096,160 7/1963 Puyear 117/107.2 X 3,163,553 12/1964Commanday et a1 117/107.2 X 10 Claims, N0 Drawings DIFFUSION-COATING OFNICKEL-BASE SUPERALLOY ARTICLES The present invention relates generallyto the protective coating art and is more particularly concerned with anew diffusion-coating method of providing oxidation resistant aluminumalloy coatings on nickel-base superalloy bodies, and with a novelcomposition for use in carrying out that method.

It has been known for some time in the art that by a diffusion coatingoperation protective coatings can be provided on superalloy componentsof gas turbine engines and similar parts intended for service incorrosive atmospheres at high temperatures. Thus, an article to becoated is heated in a bed of charge material and a suitable carrier inparticulate form until a coating of the desired thickness has beenformed over the surface of the article to be protected. Finely-dividedaluminum fluoride may serve as the carrier while the charge material isan iron-base alloy containing 19 to 35 percent aluminum, whichcodeposits with the iron and diffuses into the surface of the superalloyarticle producing a smooth and bright coating of good oxidation anderosion resistance. Such coatings, however, must be comparatively thickto provide the protection desired and consequently are relativelyexpensive to produce and are not readily applicable to articles havingclose dimensional tolerances.

I have discovered that the protection afforded by such coatings can beobtained with different and much thinner coatings which can be producedmuch more rapidly and economically than those of the prior art. Thediffusion coating technique can therefore now be extended tohigh-precision articles such as jet engine blades. Further, I have foundthat contrary to the prior art teachings, it is unnecessary to codepositiron and aluminum to avoid either coating irregularity or corrosion ofthe workpiece, and it is also not necessary to employ as the coatingsource or charge material an alloy of iron and aluminum containingsubstantially less aluminum than iron. In fact, it is essential to thenew results of this invention that the charge material contain 50percent to 60 percent of aluminum and that the process be carried out inthe presence of hydrogen, which I believe performs a carrier function.Additionally, I have found that the charge material will desirably be ofsubstantially smaller particle size than that heretofore preferred inpractice, and will comprise a relatively small proportion of thediffusion-coating bed, being admixed with an inexpensive inert materialsuch as alumina so that sticking of the sintered charge is prevented andthe cost of the charge is reduced to the point that the economicnecessity of recovery and reuse is eliminated.

The present invention is based upon these discoveries and has bothcomposition and method aspects. Defined most broadly in method terms,this invention consists in the step of heating a nickel-base superalloyarticle to be provided with a protective coating to the temperaturerange of 900 to 1, 100C in the presence of hydrogen, an aluminum sourceand a halide carrier in particulate form. More specifically, thisprocess includes as additional and preliminary steps the mixing of thealuminum source and carrier in the form of minus 100- mesh particle sizeto provide a bed and the placing of the superalloy article to be coatedby the diffusion coating process within that bed. Preferably, thecarrier is sodium fluoride or ammonium fluoride and the aluminum sourceis FeAl Fe Al or FeAl or a mixture thereof. Also preferably, thealuminum source and carrier of minus 325-mesh size are admixed withalumina of similar particle size to provide a diffusion-coating bed madeup of from five to 40 percent of the aluminum source, from 0.1 to 0.4percent carrier and from about 60 to percent alumina.

In its compositional aspects, my invention concept in general termstakes the form of a substantially uniform mixture of minus IOO-mesh sizefines of alumina, aluminum source and halide carrier in which thealuminum source comprises from 5 to 40 percent of the mixture and thecarrier is present in amount from 0.1 to 0.4 percent, the balance beingalumina. In my preferred practice, however, the aluminum source is FeAIFe- Al or FeAl and is present in amounts of about 40 percent in the caseof either FeAl or Fe Al and about eight per cent in the case of FeAlAlso, in these compositions or mixtures, the carrier is preferablyammonium fluoride or sodium fluoride and is present in the mixture inamount about 0.2 percent.

As indicated above, the process is conducted at a temperature from 900Cto 1,100C with l,050C at present representing the best practice of theinvention. The results which are obtained in using temperatures outsidethis range, and particularly below the lower end of it, are not asconsistently good as usually desired, while temperatures above the upperend of the range do not afford a sufficient advantage of greaterdiffusion-coating rates or efficiencies to warrant the increasedequipment and operating costs.

As the activator or carrier material of the system, any convenienthalide or halide mixture may be used in this process. As a practicalmatter, however, I prefer to use either ammonium halide or an alkalimetal halide. Another halide such as aluminum chloride or aluminumfluoride may be used although it is neither as economical or easy to useas the corresponding ammonium or sodium or potassium salts.

The thickness of the diffusion coating produced in accordance with thisprocess will depend mainly upon the temperature at which the process iscarried out and the length of the diffusion-coating period. Because ofthe high degree of uniformity of the resulting diffusion coatings andthe freedom of these coatings from breaks and flaws, I prefer to limittheir thickness to a total of 10 to microns. Coatings of such thicknesswill provide oxidation resistance to the extent that only much thickercoatings of the prior art afford, and there is normally no significantadvantage in prolonging the process to obtain diffusion coatings of suchgreater thickness.

In actual experiments conducted in accordance with the best presentpractice of this invention, I have produced aluminide diffusion coatingson a number of nickel and nickel-base superalloy articles using ammoniumfluoride or sodium fluoride as the carrier and FeAI or Fe Al or FeAl asthe aluminum source or charge material. In every run, alumina was usedas an inert filler to make up a treating bed consisting of asubstantially uniform mixture of about 0.2 percent of carrier, fromeight to 40 percent charge alloy and balance alumina, all of particlesize of minus IOO-mesh and in several instances minus 325-mesh. Thetreating temperature was maintained substantially constant in every casefor a full three-hour period and in most runs was l.050C. The articlesto be coated were buried in the bed contained in a box-like retort, thenthe retort was flushed with pure, dry hydrogen (dew point approximating80C) and closed to maintain a hydrogen atmosphere substantially freefrom air throughout the treating period. The retort was then brought totemperature in an electric oven and furnace-cooled beginning at a time 3hours later. Thereafter, the nickel or nickelbase superalloy parts wereremoved from the retort and examined with the results set forth in thefollowing tables:

TABLE I Charg Alloy Run Alloy Charge Particle No. Coated Carrier AlloySize 3760 Ni .2% NH F 40% FeAl 325 3771 i do. 40% Fe Al 325 3795Superalloy do. 40% FeAl 325 MM246 3833 MMZ46 do. 40% Fe Al 325 3839 Rene80 do. 40% FeAl lO 3865 do. do. 4.0% FeAl 100 3867 do. do. 80% FeAL, l003868 do. do. 16.0% FeAl 100 3871 do. do. do. l00 3892 do 2% Nl-LC] 8.0%FeAl -l00 3894 do .23% NaF do. 100 3910 do 2% NH F do. l00 3912 do. do.do. -l00 3918 do do. 8.0% Fe Al 325 3919 do do. 32.0% Fe Al, 325 3935do. do 80% FeAl -325 TABLE II Coating Time Thickness Unit wt. gain RunNo. "C (Hours) (Microns) Mg/Cnt 3760 1050 3 43 7.5 3771 do. do. 40 6.03795 850 do. 38 3.0 3833 1050 do. 53 6.0 3839 950 do. 3l 3.8 3865 1050do. 20 2.2 3867 do. do. 30 3.0 3868 do. do. 35 3.4 3871 do. do. 31 3.53892 do. do. 34 3.7 3894 do. do. 37 3.8 3910 950 do. 24 3.0 3912 1050do. 39 4.2 3918 do. do. 36 3.7 3919 do. do. 39 4.2 3935 do. do. 41 3.6

lnevery instance the coating on the test specimen was uniform inthickness and consisted of nickel aluminides of aluminum contentdecreasing with depth in the coating. Bonding of the coatingconsequently was consistently good and the coatings were all continuousand hole-free and resistant to oxidation at elevated temperature. Thegain in weight in all cases is attributable to the deposition ofaluminum which is in contrast to the prior art practice of carrying outthe diffusion-coating operation in such a way as to codeposit iron andaluminum.

The hydrogen pressure within the retort before the heating step is begunis not critical in terms of this diffusion-coating method and neither isthe presence in the bed of an excess of carrier. However. in order toavoid the development of undesirably high gas pressures during thehigh-temperature stage of the process, I prefer to limit the carrier toless than one per cent of the bed and to limit the initial hydrogenpressure in the retort to 10 psi gauge pressure.

in the specification and the appended claims whenever proportions orpercentages are stated. it is with reference to the weight basis. Also,reference is to the Tyler Standard Screens in all practice sizedescriptions.

What 1 claim as new and desire to secure by Letters Patent of the unitedStates is:

1. The diffusion-coating method of providing an aluminum-containing,oxidation-resistant protective coating on a nickel-base superalloyarticle which comprises the steps of providing a diffusion-coating bedof a mixture of-l00 mesh size particles of from 5 to 40 percent of analuminum source and from 0.1 to 0.4 per cent ofa halide carrier and fromabout 60 to percent of a filler material said aluminum source beingselected from the group consisting of FeAl I e- A1 and FeAl and mixturesthereof and said halide carrier being selected from the group consistingof ammonium fluoride, aluminum chloride and sodium fluoride. embeddingthe superalloy articles in the diffusion-coating bed, flushing the bedwith hydrogen, and heating the bed and article therein to 900 to 1,100Cwhile maintaining a hydrogen atmosphere in the bed until a coating ofthe desired thickness has been formed on the article.

2. The method of claim 1 in which the said source is FeAl 3. The methodof claim 1 in which the carrier is sodium fluoride.

4. The method of claim 1 in which the carrier is ammonium fluoride.

5. The method of claim 1 in which the aluminum source is Fe Al 6. Themethod of claim 1 in which the aluminum source is FeAl;;.

7. The method of claim 1 in which the aluminum source is a mixture ofFeAl Fe Al and FeAl:,.

8. For use in the diffusion-coating of a nickelbase superalloy article asubstantially uniform mixture of minus l00-mesh size fines of alumina,an aluminum source selected from the group consisting of FeAl Fe- Al andand FeAl and mixtures thereof, and a halide carrier selected from thegroup consisting of ammo nium fluoride, ammonium chloride, and sodiumfluoride, the aluminum source being present in the mixture in amount offrom live to 40 percent, the said carrier being in amount from 0.l to0.4 percent and the alumina constituting the balance of the mixture.

9. The mixture of claim 8 in which the aluminum source is FeAlwhichamounts to 40 percent of the mixture, and in which the carrier isammonium fluoride amounting to 0.2 percent of the mixture.

10. The mixture of claim 8 in which the aluminum source is FeAlamounting to eight percent of the mixture and the carrier is sodiumfluoride amounting to about 0.2 percent of the mixture.

2. The method of claim 1 in which the said source is FeAl2.
 3. Themethod of claim 1 in which the carrier is sodium fluoride.
 4. The methodof claim 1 in which the carrier is ammonium fluoride.
 5. The method ofclaim 1 in which the aluminum source is Fe2Al5.
 6. The method of claim 1in which the aluminum source is FeAl3.
 7. The method of claim 1 in whichthe aluminum source is a mixture of FeAl2, Fe2Al5 and FeAl3.
 8. For usein the diffusion-coating of a nickelbase superalloy article asubstantially uniform mixture of minus 100-mesh size fines of alumina,an aluminum source selected from the group consisting of FeAl2, Fe2Al5and and FeAl3 and mixtures thereof, and a halide carrier selected fromthe group consisting of ammonium fluoride, ammonium chloride, and sodiumfluoride, the aluminum source being present in the mixture in amount offrom five to 40 percent, the said carrier being in amount from 0.1 to0.4 percent and the alumina constituting the balance of the mixture. 9.The mixture of claim 8 in which the aluminum source is FeAl2 whichamounts to 40 percent of the mixture, and in which the carrier isammonium fluoride amounting to 0.2 percent of the mixture.
 10. Themixture of claim 8 in which the aluminum source is FeAl3 amounting toeight percent of the mixture and the carrier is sodium fluorideamounting to about 0.2 percent of the mixture.